1. Field of the Invention
The present invention relates to a projection color television tube, and more particularly to an electron gun for such tube having improved high voltage performance.
2. Description of the Related Art
A projection color TV uses three monochrome CRTs to produce the primary colors, i.e., red, blue and green, which are superimposed on a large projection screen to produce a full color display image. Because the images on the tube screens are not viewed directly, but are magnified and projected by a system of projection lenses, the individual cathode ray tubes are driven at higher voltages and beam currents than would be encountered for direct view tubes, in order to produce a full color display of acceptable brightness. U.S. Pat. No. 4,904,898, issued to Penird et al. discloses an electron gun assembly where a focusing electrode is coaxially surrounded by an accelerating electrode. A figure of U.S. Pat. No. 4,904,898 is reproduced here as FIG. 1. It shows an electron gun having a cathode 21 to emit electrons, a control electrode 22, and a screen electrode 23. The electron gun further includes a first focus electrode 24 and a second focus electrode 25. The end portion of the electron gun is coaxially surrounded by an accelerating electrode 26. More particularly, the second focus electrode 25 has a cup-shaped portion, which is comprised of a short cylindrical portion 25b and a tapered portion 25c, integrally connected to a cylindrical base portion 25a. Tapering the wall of the second focus electrode 25 helps to reduce a surface comprising the high voltage gap between the second focus electrode 25 and the accelerating electrode 26. It further increases the distance between the interior surface of the accelerating electrode 26 and the outer surface of the second focus electrode 25. The disclosed electrode structure is stated to have good high-voltage performance. However, because the second focus electrode 25 still has a cylindrical portion 25b at one end thereof, there remains a possibility of a particle or projection becoming a field emitting site. There has been no attempt to taper the focusing electrode all the way to the end thereof presumably because it was believed that it is necessary to the main electronic lens.
To solve the above problem, an object of the present invention is to further improve the high voltage performance of an electron gun for a monochrome cathode ray tube without degrading its lensing performance.
Another object of the present invention is to provide an electron gun for a monochrome cathode ray tube which can be effectively high voltage conditioned.
In accordance with the invention, it has been discovered that the high voltage performance of the monochrome cathode ray still can be achieved by modifying the shape of the focusing electrode such that the end of the focusing electrode is also tapered.
In accordance with the invention, the side walls of the focusing electrode is tapered inwardly from the end.